The invention relates to apparatus for the chemical analysis of samples wherein neutral particles are ejected from the surface of the sample by ion bombardment, ionized in a plasma, and analyzed by mass spectrometry.
A method and apparatus of this type are known from PHYSICS LETTERS, vol. 3 (1972), pp. 211-212, and APPLIED PHYSICS, vol. 14 (1977), pp. 43-47. The method, known as SNMS (Sputtered Neutral Mass Spectrometry), offers the advantage over the SIMS (Secondary Ion Mass Spectrometry) method that, in the process of ionization of the particles to be analyzed, the nature of the sample surface exerts no influence. The SNMS method therefore permits quantitative determination of the chemical composition of the sample. With the SIMS method, the formation of secondary ions at the sample surface is dependent, in a complicated manner, upon the chemical nature of the surface, and variations in sensitivity of several powers of ten may occur. For this reason, while the SIMS method often permits high-sensitivity qualitative analyses to be made, quantitative analyses usually are not possible with this method.
In the known SNMS method, a high-frequency plasma, preferably an argon plasma, disposed in front of the sample has two functions. One of these is the production of primary ions which are accelerated onto the surface of the sample when a voltage is applied to the sample. The other function is the post-ionization of the neutral particles thereby ejected from the sample which then are detected by means of a mass spectrometer of any type, for example a quadrapole mass analyzer followed by a secondary electron multiplier or a Faraday cup. One disadvantage of the known SNMS method is that a high post-ionization probability of the neutral particles to be analyzed is always tied to a high bombardment-current density at the sample, and hence to a high removal rate. Another, is that it is not possible to achieve high lateral resolution since the primary ions of the plasma cannot be directed onto well-defined areas of the sample.